Investigation into the photochemical bleaching of pulp utilizing various light sources.

Abstract

Since pulp and paper as products are commodities, the pulp and paper industry are constantly looking for any advantage that will keep production costs down and improve profitability. One of the main sources of cost in any pulp and paper mill is the vast quantity of energy used. Thus, any alternative or disruptive technology that reduces energy usage or costs would provide a strong incentive for research. Photochemical bleaching is one such potentially disruptive technology. In this research, three potential light sources for the photochemical bleaching of pulps were investigated, namely, solar radiation, microwave discharge electrodeless lamps (MDELs), and ultraviolet light emitting diode arrays (UV LED arrays). Of the three, only the UV LED array was shown to be both viable and practical. The photochemical bleaching of a post-oxygen stage Cloquet Aspen kraft pulp was successfully undertaken with solar radiation as a light source for both pure caustic soda and alkaline peroxide as bleaching reagents. The addition of metal oxide additives or the semiconductor photocatalyst titanium dioxide gave poorer photobleaching performance due to the increased opacity of the pulp suspension and the low intensity of the radiant light source. Given that a full five days were required to achieve a small photobleaching effect, the use of solar radiation, although viable, is not practical as a technology. The ignition and sustained “burning” of a Primarc MDEL were shown to be random and erratic due to the lack of calibration and tuning of the Roto SYNTH microwave oven’s irradiation chamber for the Primarc MDEL. Of the various reactor designs investigated, the suspended MDEL “bucket” reactor was shown to be the best performer and most viable alternative. However, investigation of the photochemical bleaching of Cloquet unbleached kraft paper-grade pulp with the suspended MDEL “bucket” reactor design and the Roto SYNTH microwave reactor oven for microwave irradiation, gave no statistically significant differences between the controls without a MDEL and the MDEL irradiated experiments. The photochemical bleaching of Cloquet unbleached kraft paper grade pulp with the UV LED array was successful when using only pure caustic soda, alkali peroxide and alkali oxygen as bleaching reagents. As regards to the overall bleaching performance, the alkali peroxide and alkali oxygen options performed similarly while the pure caustic soda option performed slightly more poorly. Based on the literature reviewed, the hydroxyl radicals generated in the photobleaching process were the active bleaching species in all three cases. In an extension of the alkali peroxide evaluation, the impact of pulp suspension pH, temperature and consistency, as well as the UV radiant intensity level (i.e. UV LED array power levels) were further evaluated. In addition, various photocatalysts were investigated for the alkali peroxide option, as well as various photosensitizers for the alkali oxygen option. Successful photochemical bleaching occurred across the entire pH range tested (i.e. pH 2.5 to 11.0) with better performance at the alkaline pH range with a maximum performance at pH 10.5. Photochemical bleaching increased with temperature from 25 to 43 °C, whereafter it plateaued up to 75 °C. Increased pulp suspension consistency from 0.5 to 1.5% gave a reduction in photochemical bleaching performance, whereafter it plateaued up to a consistency of 3.0%. Photochemical bleaching increased linearly with illuminant intensity as the UV LED array power levels were increased from 10 to 100%. Of the photocatalysts tested, only titanium dioxide and cerium dioxide were successful in improving the alkali peroxide photobleaching of pulp at the charge levels used (5.0 and 20.0 kg t-1 respectively). None of the photosensitizers tested (i.e. methylene blue and rose bengal) operated successfully for the alkali oxygen photobleaching of pulp in the experimental work undertaken. The photochemical bleaching of various starting pulps (Cloquet kraft unbleached, Ngodwana PHK unbleached, Stanger unbleached, Saiccor acid sulfite unbleached, Gratkorn magnefite unbleached, and Somerset continuous kraft unbleached) was successful for all starting pulps. However, bleaching performance varied from pulp to pulp. A comparison of the thermal elemental chlorine free (ECF) and total chlorine free (TCF) sequences with the thermophotochemical TCF sequence for bleaching paper grade pulps, showed that the thermal ECF and thermophotochemical TCF sequences were both successful in achieving the target brightness (i.e. 89%) while the thermal TCF sequence underperformed slightly (i.e. 85%). The thermophotochemical TCF sequence gave acceptable pulp properties for viscosity, hand sheet pulp strength, and fibre morphology as compared to the thermal ECF sequence as a control sequence, while the thermal TCF gave significantly weaker pulps. For bleaching dissolving grade pulp, both the thermal ECF (as a control sequence) and the thermophotochemical TCF sequences were successful in achieving target brightness and above (i.e. 91%). Both sequences gave final viscosities within the target range (500 to 560 ml g-1). In addition, both sequences gave acceptable carbohydrate sugar contents, alkaline solubilities, and Sappi quick reactivity indexes. Lastly, both pulps also had similar fibre morphologies showing a reduction to shorter fibre lengths due to chemical refining during both bleaching processes. “Pseudo” second order reaction kinetics were established for the oxidants: alkaline peroxide, pure caustic soda, and alkaline oxygen for photochemical bleaching utilizing the UV LED array reactor, with strong kinetic relationships demonstrated for pulp extent/rate of delignification and brightness development, but, a weak relationship for pulp viscosity. Results demonstrated that the three oxidants utilized different chemical mechanisms for generating the hydroxyl radical species employed for bleaching, with the best results obtained for the hydrogen peroxide and the poorest for the pure caustic soda. An observed discrepancy between the “pseudo” first order reaction kinetics in Marcoccia’s investigation and the “pseudo” second order reaction kinetics in this study were due to differences in the conditions, apparatus and charges employed in both. The final conclusion drawn from this study is that photochemical bleaching of pulp, using an ultraviolet light emitting diode array at 365 nm, can serve as a viable alternative to thermally-driven bleaching provided certain technical issues can be resolved (i.e. operational consistency and comparative energy efficiency of thermal- versus photo-bleaching).